In traditional film/screen medical diagnostic imaging, an anatomical part of a patient is positioned between an x-ray source and an unexposed film/screen, an x-ray exposure of the anatomical part produces a latent x-ray image in the film and the film is developed. The developed x-ray film is then viewed on a light box by a diagnostician (radiologist/physician). More recently, medical diagnostic imaging modalities, such as CT and MRI scanners, have produced video x-ray images which are viewed on a video monitor. Frequently, a permanent x-ray image of a video x-ray image is made by photographing the video monitor to produce an x-ray film image. The x-ray film image can also be produced by a laser printer (such as the KODAK EKTASCAN LASER PRINTER sold by the Eastman Kodak Company, Rochester, N.Y.).
When using a laser printer to produce a permanent x-ray film image, problems arise from the different video formats used by different medical imaging modalities. Typically, the analog video signal produced by the modality is converted to a digital image signal which is used to drive the laser printer. In order to match the different analog signal values produced by different modalities to the predetermined signal range of the digitizing circuit, it is desirable to dynamically shift the level of the analog signal before digitization. The DC restoration circuits disclosed in the following patents are representative of known circuits which are not entirely suitable for such applications.
U.S. Pat. No. 4,827,191, issued May 2, 1989, inventor Chapman.
U.S. Pat. No. 4,873,702, issued Oct. 10, 1989, inventor Chin.